An ultrasonic sensor is mounted to an automobile (automotive vehicle), for example. A sending portion in the sensor sends ultrasonic waves to an object to be detected. The object reflects the waves, and a receiving portion in the sensor receives the reflected waves. Thus, a position or a distance for the object around the automobile can be measured. That is, the ultrasonic sensor is used for a safe driving by monitoring surrounding objects of the automobile.
For example, an automatic parking supporting system using an ultrasonic sensor is practically used. In the system, a back-sonar is used for detecting a human or an obstacle existing in the back of an automobile. The ultrasonic sensor is mounted to a rear part of the automobile, and receives ultrasonic waves reflected by the human or the obstacle. Thus, a back side collision with the human or the obstacle can be reduced.
Further, by using a micro electro mechanical system (MEMS) technology, an oscillating portion made of a piezoelectric membrane is formed on a membrane portion in a substrate, as an element for an ultrasonic sensor. Here, when the element is mounted to an automobile in an exposed state, a distance to an object to be detected may not accurately be measured, because water drops or dusts may easily adhere onto a surface of the element. Moreover, the element may be damaged and destroyed by a load of an external force, e.g., a collision with a small stone.
JP-A-2002-58097 discloses an ultrasonic sensor having a protective construction for reducing the adhesion and the destruction. In the sensor, a receiving element is disposed in an aluminum case so as not to be exposed to an outside, and a piezoelectric oscillator for detecting ultrasonic waves is directly mounted to an oscillating board. The sensor can receive ultrasonic waves by oscillations, i.e., vibrations, of the oscillating board.
However, a mechanical strength of the receiving element produced by the MEMS technology is low due to its structure. When the element is directly mounted to the oscillating board, the element may easily be damaged. By contrast, if a space is provided between the piezoelectric membrane and the oscillating board in order to reduce the damage, ultrasonic waves may not effectively be transmitted to the element.